1. Field of the Invention
The present invention relates to a method and apparatus for soldering printed circuit boards under reflowing of solder preforms or solder pastes applied onto the boards in a heating zone (this mode of soldering will be referred to hereinafter as "reflow-soldering"). More particularly, the present invention relates to a method and apparatus for reflow-soldering printed circuit boards carrying electronic parts with lead wires of relatively low temperature resistance alone or together with electronic parts of relatively high temperature resistance such as chip parts and flat packages (referred to hereinafter sometimes simply as SMD) in a specific manner and a specifically designed heating zone without causing any soldering failure and any environmental pollution.
2. Description of the Prior Art
From the past, a soldering operation of printed circuit boards having mounted thereon SMD of relatively high temperature resistance was carried out in a reflow-soldering apparatus where the soldering operation is made in a high temperature atmosphere maintained at a temperature of 210.degree.-250.degree. C. In case of printed circuit boards having mounted thereon SMD of relatively high temperature resistance together with electronic parts with lead wires having a relatively low temperature resistance, the soldering operation was carried out by mounting SMD to a printed circuit board to which a solder paste had been applied, subjecting the board to soldering in a reflow-soldering apparatus, mounting the electronic parts with lead wires to the board, soldering the board in a jet-type soldering tank or a flow-dip type soldering tank after painting a flux to the board and subjecting it to a preliminary heating treatment, and washing the board with freon (marketed from DuPont) or the like solvent.
In accordance with the recent versatility in surface-mounting technique and in accordance with the reduction in cost of printed circuit boards, electronic parts with lead wires have been soldered heretofore in a flow-dip type soldering tank or in a jet-type soldering tank. Among these electronic parts, however, those of poor temperature resistance as low as 120.degree. C. have not yet been soldered in a reflow-soldering apparatus. In the currently used reflow-soldering apparatus, the temperature of the heating zone is uniformly maintained at about 210.degree.-150.degree. C. Accordingly, there is raised a problem such that a reflow-soldering apparatus is unsuited for soldering electronic parts with lead wires which are poor in temperature resistance as low as about 120.degree. C.
In order to solder electronic parts with lead wires in a jet-type soldering tank or a flow-dip type soldering tank, additional devices such as a flux-coating device, a preliminary heating device and a washing device become necessary. Among these additional devices, the washing device wherein freon is used as a washing solvent may cause a serious problem; when the vapor of freon leaks out of the device, such wasteful consumption of the solvent not only increases the cost of the products but also causes bad working environment and even pollution. Thus, exclusion of the use of freon is highly desired for soldering electronic parts with lead wires. Further, the use of a reflow-soldering apparatus is also desired for soldering to printed circuit boards both electronic parts having a high temperature resistance and electronic parts having a low temperature resistance.
Under the above mentioned circumstances, there is a great demand for developing a new method and apparatus for carrying out soldering of electronic parts with lead wires having a poor temperature resistance by means of reflow-soldering not accompanying any washing step with freon.